Class-D audio amplifiers are often used for audio amplification because of their power efficiency. Typically, the Class D audio amplifier is operated in the switch mode with minimized internal power consumption. It produces a rectangular wave at the output stage that is filtered before delivered to a load. The filtered signal wave is an amplified version of the input signal wave. Class D audio amplifiers are usually used for high power applications. For low power applications, Class A/B amplifiers are still popular.
When the input audio signal exceeds the audio amplifier's linear range, the output of the amplifier saturates. Oscillations at the audible band are often observed when the amplifier enters the saturation condition and exits the saturation condition, as indicated in FIG. 3. This may result in the “clipping” of audible noises. This problem is more severe in the class D audio amplifier because the switching power supply can skip switching cycles due to the minimum on and off time constraints. If the power supply skips sufficient cycles, the effective operation frequency may enter the audible frequency range and induce unexpected audible noises.
There are several known methods to resolve the problem. The first method is to limit the amplitude of the input signal with a clamping circuit. However, without information on the audio source's output impedance, this may not be practical and can degrade the audio signal quality. The second method is to add an automatic gain control (AGC) pre-amplifier before the input of the class-D audio amplifier. This AGC pre-amplifier limits the input signal amplitude to prevent the output saturation, but the implementation is rather complex and adds a significant cost. The limitation may get more severe for low frequency audio signals. The third method is to add a high-pass filter to limit the minimum audio frequency passing into the class-D amplifier, but may not solve the problem completely.
Accordingly, more improvements are needed to reduce audio noise near saturation in the class D audio amplifier.